In conventional practice, refined copper is produced from sulfide concentrates by pyrometallurgical operations leading to the production of blister copper which is electrolytically refined. The various impurities associated with copper in the concentrate are removed during these operations which could typically include smelting in reverberatory, electrical or flash furnaces followed by oxidation of the resulting matte by conversion and then anode furnace refining prior to casting anodes for electrolytic refining.
About 15 percent of the copper produced annually is derived from oxide ores. In such cases it is possible to use a hydrometallurgical process involving acid leaching in agitated tanks, vats or dumps followed by solvent or liquid-liquid extraction. The latter provides a pure concentrated solution of copper sulfate from which the pure metal is won by electrodeposition onto cathodes in electrolytic cells having insoluble anodes. This technology is well described in Extractive Metallurgy of Copper, A. K. Biswas and W. G. Davenport, Pergammon Press, New York, 1980, whose disclosure is incorporated by reference herein.
In the past 20 years, there has been a marked increase in the demand by wire mills for the copper produced annually in the USA and this application is the most demanding in terms of cathode quality. In order to be reduced to wire, the cathode has to be melted before casting and hot rolling and during these operations careful control must be exercised to ensure that the original quality of the cathode is not impaired. For the melting operation, shaft furnaces are generally superior to other types in minimizing impurity levels, and continuous rod drawing systems have now almost entirely replaced wire bar static casting followed by batch hot rolling.
Acceptable grades of copper for magnet wire drawing have to conform to rigid chemical specifications and to standards for physical properties such as conductivity, mechanical integrity and annealability set by wire manufacturers. The finer the gauge of the wire the more stringent these specifications become and failure to meet these specifications results in loss of production by frequent breakage. These requirements have been described in "Quality of Cathode Copper," L. K. Bigelow and I. S. Servi, AIME Annual Meeting, New York, Feb. 19, 1975, whose disclosure is incorporated by reference herein. Especially important is to provide very low levels of S, Pb, Sb, As, Se, Te, Bi and Ag, on the order of a few ppm of each.
In recent years there has been considerable interest in the development of hydrometallurgical processes for treating sulfide concentrates in order to overcome difficulties and high costs in meeting stringent environmental standards set for pyrometallurgical smelters. A wide range of such processes has been seriously considered in recent years as discussed in "Exploitation of Complex Sulfide Deposits: A review of processing options from ore to metals," G. Barbery, A. W. Fletcher and L. L. Sirois, Complex Sulfide Ores, Institute of Mining and Metallurgy, London, 1980, whose disclosure is incorporated by reference herein. This article describes three sulfate based processes and eight chloride based processes which have been tested on a laboratory or pilot plant scale. Yet another seven processes are described in the study carried out by Charles H. Pitt and Milton E. Wadsworth for the U.S. Department of Energy, Division of Industrial Energy Conservation, (Contract No. EM-78-S-07-1743) and published in "An Assessment of Energy Requirements in Proven and New Copper Processes," University of Utah, December 1980, whose disclosure is incorporated by reference herein. It must be emphasized that in these eighteen different processing routes, the final copper metal product is obtained after a series of unit operations which differ in the basic chemistry employed. It will not be surprising therefore to find that the trace impurities in the final product will differ from case to case and certainly from conventional smelting and refining practice.
There is a need therefore for an alternative refining process which will efficiently and economically refine the new copper products from these developing hydrometallurgical operations as well as other processes. This will be particularly important for processing routes which do not include an electrolytic refining step which, among other examples, separates copper from silver. Such processes include the CLEAR, Cymet, Elkem, Dextec, Envirotech Corporation and the University of Utah Martin-Marietta processes. See, Barbery et al and particularly with respect to CLEAR, G. E. Atwood and R. W. Livingston, "The CLEAR Process, A Duval Corporation Development," Erzmetall 33 (1980) No. 5 and U.S. Pat. Nos. 3,785,944 and 3,879,272, whose disclosures are incorporated by reference herein.
One potential method of achieving the necessary refinement of copper, and of other metals as well, involves distillation, especially by electron beam irradiation. The technique of separating the components of a mixture of metals by the selective distillation of those having lower boiling points and higher vapor pressures at the operating temperature, dates back to 1913 when the celebrated Langmuir equation was published relating the rate of evaporation of an element or pure substance into a vacuum to the evaporation temperature. The rate is shown to be proportional to the activity coefficient of the element being evaporated. This is simply a recognition of the likelihood of deviations from ideality in any given system. Thus, Raoult's Law predicts that, ideally, the vapor pressure of a given species in a mixture will be given by the product of the mole fraction of that species and its vapor pressure in the pure state. The activity of the species, on the other hand, is given by the ratio of the actual vapor pressure of the species to the vapor pressure of the species in the pure state, all parameters, of course, being taken at the same temperature. Hence, the activity of a given element in the system of interest provides a measure of the deviation from ideality at a given temperature.
In practice, ideal behavior has been observed in some systems; however, in most systems, negative or positive deviations are observed. For example, in the bismuth/tin system, the elements follow Raoult's Law--ideal behavior is observed. In the lead/zinc system, positive deviations from ideality are observed, i.e., activities greater than 1 have been measured, usually due to miscibility gaps. In the magnesium/bismuth and zinc/copper systems, negative deviations from ideality have been observed, usually due to the formation of relatively nonvolatile compounds, i.e., the effects of intermetallic interactions serve to provide an energy barrier to the expected evaporation of the more volatile species.
As can be seen, a priori, it is not possible to predict whether a given metallic species can be removed from a bulk metal based simply on a comparison of the relative volatilities of the two metals in the pure state. When removal of relatively small amounts, e.g., traces of impurities from a bulk metal is of concern, the situation is significantly exacerbated and more unpredictable. For example, as discussed by Bigelow et al, supra, page 13, "the specific effect of a trace element is most powerful when the first small amount is added; thereafter, the effect continuously decreases until the solubility limit is reached." In evaporating an impurity from a bulk metal, as the concentration of the impurity continuously decreases, the average degree to which the remaining impurity atoms are bound by bulk metal atoms must be expected to significantly increase at least at some stage. Hence, in binary metallic systems, distillability is a variable and unpredictable process as a function of concentration. Moreover, the successful distillation of an impurity from a metal at relatively high contents of that impurity is not a sufficient basis for reliably predicting that the impurity will still be distillable from the metal at lower concentrations.
The situation becomes extremely complex when more than one impurity is involved, i.e., in ternary or higher systems. As also recognized by Bigelow, supra, page 13, "various trace elements may interact with each other, for instance, silver with selenium and lead with sulfur," with particular reference to the presence of these metals in copper. Furthermore, unlike the binary system where the first small amount causes, proportionately, the greatest effect, when considering interactions between and among impurities per se, the opposite effect is to be expected. That is, in binary systems, the first impurity atom encounters only the effects of bulk metal atoms. Subsequent impurity atoms see an increasingly diluted effect of the bulk metal atom and dimer interactions can be ignored at least intially. In ternary and higher systems, as the amounts of the two or more impurities are increased upward from the first such atoms, the probability of effective intermetallic interactions between the impurities increases since the average distance between impurity atoms correspondingly decreases with increasing concentration. Thus, impurity atoms are more tightly "bound" by each other the higher the concentration of either, at least up to some unpredictable point at which the effect of a given concentration of impurity atoms on other impurity atoms might become saturated. As a result, despite the fact that a given impurity may have been successfully distilled from a base metal in the presence of a given impurity distribution in the metal, it is not possible to predict that the same impurity will be distillable from the same metal if the impurity distribution is changed since the spectrum of intermetallic interactions will change or, at least, a change must be expected.
As can be seen, there are many possible interactions in a given system which can lead to deviations from ideality. This makes the distillability of a given impurity from a base element having a given distribution of impurities unpredictable. The probability and severity of intermetallic interactions (e.g., compound formation) between impurities and the bulk metal generally decreases with increasing impurity concentrations. The probability and severity of intermetallic interactions between and among impurities, in contrast, will generally increase with increasing impurity concentration. In both cases, the greatest change in the level of intermetallic interactions will normally occur as a function of changes in impurity concentrations at low levels, e.g., trace elements on the order of 1-100 ppm or less. Hence, when considering a single impurity in a bulk metal, it may be possible to predict that higher concentrations of the impurity will be distillable if it has been observed that lower concentrations have been distillable, but the reverse will not be true. When considering bulk metals containing more than one impurity, the distillability of one impurity at a given concentration and in the presence of given concentrations of other impurities, will not provide a basis for predicting the distillability of the same impurity from the same bulk metal if its concentration is higher (increased impurity interactions) or lower (increased bulk metal interactions) or if the concentration of the other impurities is higher (increased impurity interactions) or lower (increased bulk metal interactions).
The complexity involved in predicting whether a given impurity will be distillable from a given metallic system has been well summarized by Gilchrist, Extraction Metallurgy, 2nd Edition, Pergamon Press (1980), page 132,
The treatment of activities in multicomponent systems is more difficult than in binary systems because interactions between atoms of different solutes usually alter their activities from the values they would have had in binary solutions at the same concentrations. In a few cases activities have been determined by experiment over large parts of ternary systems, but in general this approach is inadequate to meet the degree of complexity likely to be encountered in industrial applications; PA1 When calculating vapor pressures of the elements above their solutions in iron, it is important to consider these deviations from Raoult's Law for the effect can be very large, especially in complex alloys. PA1 wherein the vapor pressure of the condensate of said vapor phase at its condensation point is less than about 10.sup.-3 torr; PA1 and wherein the effective operating pressure of each stage is maintained at a low level compatible with electron beam irradiation;
and by Olette, "Vacuum Distillation of Minor Elements from Liquid Ferrous Alloys," Physical Chemistry of Process Metallurgy Part 2', pages 1065-1087, G. R. St. Pierre, Ed., Interscience, New York, 1961, at 1070.
In short, where the relative concentrations of impurities in a given bulk metal system are different from those for which data are available, it is not possible to predict the distillability of a given impurity element in the new system.
The copper system provides an excellent example of the complexity encountered in metal systems of interest in industry. Hydrometallurgically processed copper typically contains impurities including selenium, tellurium bismuth, lead, sulfur, silver, antimony, arsenic, zinc, iron, tin, nickel, oxygen, inter alia. The levels of these impurities can vary significantly from process to process and from ore sample to ore sample. Most of these elements have been shown to form intermetallic compounds with copper in at least some temperature range. In addition, there are very many intermetallic compounds observed and/or predicted between impurity species, inter alia, Ag.sub.3 Sb, Sb.sub.2 Te.sub.3, As.sub.2 Te.sub.3, AsTe, Sb.sub.2 Se.sub.3, As.sub.2 Se.sub.3, AsSe, PbTe, PbSe, Ag.sub.18 Te.sub.11, Ag.sub.2 Te, Ag.sub.2 Se, SeO, SeS, TeO, TeS, BiO, BiS, SbO, SbS, AsO, AsS, etc. See, e.g., Nagamori et al, Metallurgical Transactions B, Volume 13B, September 1982, 319 and the F*A*C*T SYSTEM program by W. T. Thomspon, A. D. Pelton, C. W. Bale, FACT SYSTEM, Montreal, Quebec, Canada, Copyright 1982, Thermfact LTD/LTEE. While many of these intermetallic compounds are predicted only in theory, many others have been observed. The high complexity of the impurity distribution in copper metal, especially those which have been hydrometallurgically processed, is indisputable.
Heretofore, no sufficiently effective distillations of hydrometallurgically processed copper have been reported. All distillations using conventional methods such as induction heating fail to provide the necessary low impurity levels. However, Santala et al have reported an electron beam laboratory distillation of silver from copper in otherwise highly pure binary alloys (The Journal of Vacuum Science and Technology, Vol. 7, No. 6, 1970, 22). Therein is reported the distillation of silver from copper/silver alloys containing respectively, 8%, 15.7%, 29.9% and 42.2% of silver. In all cases, relatively high amounts of silver remained in the refined alloy. The lowest retained amount of silver was 900 ppm in the experiments using the alloy containing 8% silver. The materials used to prepare the alloys were commercial purity fine silver containing 1-10 ppm of copper; less than 1 ppm of iron; less than 1 ppm of Mg; 1-10 ppm of Pb; and less than 1 ppm of Si. The starting material copper was certified OFHC (oxygen free high conductivity) copper meeting the specifications of ASTM B-170-66T which provides the following maximum impurity levels: 1 ppm of Cd; 3 ppm of P; 18 ppm of S; 1 ppm of Zn; 1 ppm of Hg; 10 ppm of Pb; 10 ppm of Se; 10 ppm of Te; 10 ppm of Bi; less than 40 ppm in total of Se, Te, Bi, As, Sb, Sn and Mn; and 10 ppm of O.sub.2.
Hence, Santala et al provide evidence that silver can be distilled from copper in the relatively high silver concentration range of 900 ppm and above, even in the presence of the mentioned impurity spectrum permitted in OFHC copper. But in accordance with the foregoing, it is clear that such an experiment does not resolve a consideration of whether amounts of silver greater than 20 ppm can be distilled from hydrometallurgical copper to provide final silver concentrations of less than 20 ppm, which is a typical spec for the wire mill industry. The complexity of potential intermetallic effects in the concentration range from 900 ppm of silver to less than 20 ppm of silver precludes any expectation that silver can be distilled to the requisite low levels using the electron beam method as employed by Santala et al or any other method.
In unreported electron beam distillation experiments performed by one of the inventors of this application in the sixties and which are believed not to be effective prior art vis a vis this application, silver was distilled from a starting material copper metal containing approximately 10 ppm of silver and a total of about 20 ppm of Bi, Pb, Se, Te, and S. See "Metal Refining in High Vacuum by Distillation of Electron Beam Hearths," Hunt et al, Pacific Southwest Minerals Conference, San Francisco, Mar. 25-28, 1979, Abstract; and "Proposed Program for Sales of a Copper-Silver Separation Process Based upon Volativity [sic] Differences," handout at same conference, which briefly mention these experiments. Once again, these experiments do not resolve the problem of the distillability of silver at starting concentrations greater than 20 ppm. The increased potential for additional intermetallic interactions at the higher silver concentrations in combination with even the same impurity distribution is an additional complex factor precluding any predictability or expectation of successful distillation.
Accordingly, no experiments performed heretofore resolve or reliably predict an answer to the question of the distillability of the impurity spectrum present in industrially available unrefined copper metal including lead, selenium, tellurium, bismuth, sulfur, etc. and especially including the ever-present silver at least because the silver levels involved are greater than about 20 ppm, yet must be lowered to levels less than 20 ppm.
From the foregoing, it will be seen that electron beam treatments of metals including distillation of impurities from metals have been contemplated in the past. In this regard, see, e.g., U.S. Pat. Nos. 4,035,574; 3,607,222; 3,494,804; 3,342,250; 3,219,435; 3,210,454; 3,183,077; and 3,005,859; all of whose disclosures are incorporated by reference herein. Although a wide variety of arrangements have been utilized in normal heating techniques, including vertical column fractional distillations with reflux (see, e.g., U.S. Pat. Nos. 4,265,644; 4,043,802; 3,948,495; 3,778,044; 3,632,334; 3,484,233; and 2,823,111), the advantages of conventional fractional distillation, so familiar in conjunction with liquids such as hydrocarbons, have never been achieved in electron beam systems. To a large degree, this is due to the unique and stringent requirements of electron beam operation.
In addition, the normal vertical alignment of distillation stages is not very compatible with electron beam systems; rather, horizontal arrangement of the different stages is more practical. While horizontally arranged multistage processes have been suggested in the past (See, e.g., U.S. Pat. Nos. 2,174,559 and 1,840,946 regarding metals and U.S. Pat. Nos. 3,496,159; 2,698,287; and 3,151,042 regarding conventional liquids), even in electron beam evaporation operations (see, e.g., U.S. Pat. Nos 3,343,828 and 3,288,594 whose disclosures are incorporated by reference herein), there has never been a suggestion of how to achieve the advantageous plate-like effects of conventional fractional distillations in electron beam systems. In essence this is because the conventional technology used to achieve a reflux in normal liquid or even in melted metal systems, even in horizontal arrangements, is simply not applicable to electron beam systems because of the dramatically different requirements involved.
For example, reflux with countercurrent contact of vapor and liquid which gives the reboiling phenomenon in conventional vertical fractional distillations, is not possible in electron beam distillations. The vacuum requirements are simply too stringent as are the requirements for minimal beam/vapor interactions. Furthermore, a horizontal distillation with recycle such as that of U.S. Pat. No. 2,939,783 is also not applicable to an electron beam operation since the vapor phase itself is recycled. It has been discovered that a recycle of this nature in a multistage electron beam distillation is very unsatisfactory.